1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with an adapter plate of a fuel assembly top nozzle which has a fuel rod capture grid with an adjustable flow feature for tailoring the top nozzle pressure drop to the specific reactor core location of the fuel assembly.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend above and below the opposite ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in openings provided in the lower adapter plate of the top nozzle.
It is conventional practice to design the adapter plate of the fuel assembly top nozzle to accommodate three requirements. First, the adapter plate must satisfy a structural criteria, that is, it must be capable of lifting the fuel assembly under a 6 g load. Second, it must serve a fuel rod capture function in which the fuel rods are mechanically restrained by the adapter plate from ejections upwardly from the core. Third, from a functional standpoint, the adapter plate must have sufficient open area to permit reactor coolant flow to pass through the top nozzle with minimum pressure drop.
U.S. Pat. No. 4,427,624 proposes a composite nozzle for a fuel assembly adapted for installation on the upper or lower end thereof which is constructed from two components. The first component of the nozzle is a casting weldment or forging designed to carry handling loads, support fuel assembly weight and flow loads, and interface with structural members of both the fuel assembly and reactor. In short, the first component is designed to satisfy the structural criteria.
The second component of the nozzle is a thin stamped bore machine flow plate adapted for removable attachment to the first component. The plate is designed to limit upward movement of the fuel rods and thus ejection thereof from the core. The plate also has multiple openings or orifices of varying size and configuration to help direct coolant flow in a predetermined path through the fuel assembly and to assure that a pressure drop of predetermined magnitude will take place across the assembly. In short, the second component is intended to fulfill the two requirements of rod capture and coolant flow with minimum pressure drop.
Notwithstanding the overall acceptability of the above-described basic approach to end nozzle construction, the second flow plate component proposed in the aforesaid patent has been found incapable of achieving both functions. In order to provide sufficient coolant flow through it to attain the minimum pressure drop desired, the plate must be built with insufficient structure to perform the fuel rod capture function. Consequently, a need remains for an alternative design of a component which will satisfy the dual requirements of minimum pressure drop and fuel rod capture.